An electric vehicle (EV) is a type of vehicle that is driven by a battery powered motor, has fewer air pollution sources such as exhaust gas and noise than a conventional gasoline engine vehicle, and has advantages such as fewer failures, a long life, and simple driving operation.
An EV is classified into a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), and an electric vehicle (EV) according to the driving source. The HEV has an engine as a main power source and a motor as an auxiliary power source. The PHEV has a motor as a main power source and an engine mainly used when a battery is being discharged. The EV does not have an engine, and has a motor as its sole driving source.
In order to charge a battery mounted on the EV using a wireless charging method, it may be necessary to couple a primary coil of a charging station with a secondary coil of the EV using magnetic resonance. Also, in order to improve efficiency of wireless charging, it may be necessary to align the primary coil with the secondary coil. In a magnetic resonant wireless power transfer system, if the primary coil and the secondary coil are not aligned, the efficiency of wireless power transfer may be degraded drastically.
For example, as shown in FIG. 1, when the EV has a secondary coil (referred to as ‘Rx coil’) magnetically coupled to a primary coil (referred to as ‘Tx coil’) installed on the ground, the secondary coil should be aligned with the primary coil. Otherwise, for example, as the distance (a) between a first extension line, extended along the central axis of the Tx coil in the form of a circular ring, and the second extension line, extended along the central axis of the Rx coil in the form of a circular ring, increases the efficiency of power transfer may decrease drastically. Here, the diameter of the primary and secondary coil may be assumed to be 300 mm and the distance (d) between them may be assumed to be 105 mm.
As shown in FIG. 2, the efficiency of power transfer (represented with S-parameter S21) in the above-described wireless power transfer system having the primary coil and the secondary coil is drastically reduced from −2.5 dB to −22.5 dB, as the distance (a) between the center axes of the Tx coil and the Rx coil is increased from 0 to 450 mm by 50 mm.
Meanwhile, one of conventional alignment methods aligns a vehicle mounting a secondary coil with a primary coil of a ground assembly using a rear camera of the vehicle. On the other hand, another conventional alignment method aligns a primary coil in a movable charging pad with a secondary coil of a vehicle by moving the movable charging pad after the vehicle is parked with a speed bump in a parking area.
However, the above-described conventional techniques require an intervention of a user in the alignment of the coils, resulting in a large deviation in alignment and inconvenience to the user, and therefore, even slight coil misalignment may cause drastic system performance degradation. Therefore, because the magnetic resonant wireless power transfer system is sensitive to coil misalignment, it is difficult for the above-described conventional techniques to realize optimal power transfer efficiency, resulting in poor system stability and reliability.
Thus, in a wireless power transfer system for charging a high-voltage battery mounted on a vehicle, an efficient method of precisely aligning the primary coil of the ground assembly disposed in the charging station with the secondary coil of the vehicle assembly disposed in the vehicle is needed.